Re: [Intel-gfx] [PATCH] drm/i915: Use dma_resv_iter for waiting in i915_gem_object_wait_reservation.

On 13/10/2021 11:41, Maarten Lankhorst wrote:

No memory should be allocated when calling i915_gem_object_wait, because it may be called to idle a BO when evicting memory.

Fix this by using dma_resv_iter helpers to call i915_gem_object_wait_fence() on each fence, which cleans up the code a lot. Also remove dma_resv_prune, it's questionably.

This will result in the following lockdep splat.

<snip>

@@ -37,56 +36,17 @@ i915_gem_object_wait_reservation(struct dma_resv *resv, unsigned int flags, long timeout) {

• struct dma_fence *excl;
• bool prune_fences = false;
• if (flags & I915_WAIT_ALL) {

• struct dma_fence **shared;
  

• unsigned int count, i;
  

• int ret;
  

• struct dma_resv_iter cursor;
• struct dma_fence *fence;

• ret = dma_resv_get_fences(resv, &excl, &count, &shared);
  

• if (ret)
  

• 	return ret;
  

• for (i = 0; i < count; i++) {
  

• 	timeout = i915_gem_object_wait_fence(shared[i],
  

• 					     flags, timeout);
  

• 	if (timeout < 0)
  

• 		break;
  

• dma_resv_iter_begin(&cursor, resv, flags & I915_WAIT_ALL);
• dma_resv_for_each_fence_unlocked(&cursor, fence) {

• 	dma_fence_put(shared[i]);
  

• }
  

• for (; i < count; i++)
  

• 	dma_fence_put(shared[i]);
  

• kfree(shared);
  

• /*
  

•  * If both shared fences and an exclusive fence exist,
  

•  * then by construction the shared fences must be later
  

•  * than the exclusive fence. If we successfully wait for
  

•  * all the shared fences, we know that the exclusive fence
  

•  * must all be signaled. If all the shared fences are
  

•  * signaled, we can prune the array and recover the
  

•  * floating references on the fences/requests.
  

•  */
  

• prune_fences = count && timeout >= 0;
  

• } else {

• excl = dma_resv_get_excl_unlocked(resv);
  

• timeout = i915_gem_object_wait_fence(fence, flags, timeout);
  

• if (timeout <= 0)
  

• 	break;
  

You have another change in behaviour here, well a bug really. When userspace passes in zero timeout you fail to report activity in other than the first fence.

Regards,

Tvrtko